1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile, and a process cartridge used therefor.
2. Description of the Background
In electrophotographic image forming apparatuses, images are formed through processes of charging, exposure, development, and transfer. It is likely that discharge products are produced in the charging process and remain on a photoreceptor and that some toner particles remain on the photoreceptor without being transferred in the transfer process. These residual materials remaining on the photoreceptor are removed in an additional process of cleaning.
Generally, such residual materials remaining on the photoreceptor are removed by a rubber blade, which is cheap and structurally simple. The rubber blade is contacted against the photoreceptor so as to frictionally remove the residual materials from the photoreceptor. Therefore, both the rubber blade and the photoreceptor are abraded each other, resulting in their short lifespan. Hereinafter, the rubber blade may be called as the cleaning blade.
On the other hand, widely-used toners have a smaller particle size to meet recent demand for higher image quality. However, disadvantageously, the smaller toner particles more easily pass through the cleaning blade. In a case in which the cleaning blade causes partial vibration due to its poor dimension accuracy or assembly accuracy, small toner particles pass through the cleaning blade much more.
To prevent the above-described problem in removing small toner particles from the photoreceptor, one proposed approach involves supplying lubricant powders to the photoreceptor from a rotary brush contacted against a solid lubricant so that a thin layer of the lubricant is formed on the photoreceptor by pressure from the cleaning blade. The lubricant present between the photoreceptor and the cleaning blade prevents abrasion or deterioration thereof. The lubricant present on the photoreceptor also prevents partial vibration of the cleaning blade, thus preventing toner particles from passing through the cleaning blade.
Most of recent image forming apparatuses employ a charging roller using AC (alternate current) discharge, for charging photoreceptor, which can meet the recent demand for higher image quality. In AC discharge, an alternate current voltage is overlapped with a direct current voltage. Advantageously, such a charging roller using AC discharge also meets demand for downsizing of apparatus, and generates less oxidizing gases such as ozone and NOx. However, AC discharge is more hazardous to the photoreceptor because the photoreceptor is repeatedly exposed to positive and negative electric discharges for 100 to 1,000 times per second, compared to DC (direct current) discharge in which the photoreceptor can be completely charged by exposure to positive discharge only once. Accordingly, it is more important for image forming apparatuses employing AC discharge to protect the photoreceptor.
In view of this situation, there is a need for applying a greater amount of the lubricant to the photoreceptor as a protective agent. Hereinafter, the lubricant may be called as the protective agent. Further, there is another need for more effectively removing smaller toner particles from the photoreceptor. To respond to these needs, the cleaning blade may be required to contact the photoreceptor with a higher pressure. However, this may result in more rapid deterioration of the cleaning blade.
Alternatively, the rotary brush may be required to contact the solid lubricant with a higher pressure to supply a greater amount of lubricant. In this case, however, the scraped off lubricant powders may be larger in size. Such large lubricant powders are difficult to uniformly cover the photoreceptor.
There has been another problem that toner particles or lubricant powders passed through the cleaning blade scatter to contaminate a charging roller.
There has been a tendency that charging rollers and cleaning blades are frequently replaced with new ones whenever they have deteriorated, while various attempts have been made to lengthen lifespan of photoreceptors. Recently, longer lifespan is also required for all members in image forming apparatus including charging roller and cleaning blade, as well as photoreceptor.
Most widely-used protective agents (i.e., lubricants) are comprised of metal soaps. Such metal soap powders are disadvantageous because they considerably pass through the cleaning blade and scatter to contaminate the charging roller. In view of this situation, Japanese Patent Application Publication No. 2008-134467 proposes a protective agent comprised of a metal soap and boron nitride. According to the publication, the protective agent comprised of zinc stearate (i.e., a metal soap) and boron nitride is suppressed from scattering and abrading cleaning blades for an extended period of time. However, the problem is that boron nitride is very expensive.
In attempting to extend the lifespan of electrophotographic image forming members such as photoreceptor, cleaning blade, and charging roller, especially in a case in which a metal soap is applied to the photoreceptor, the inventors of the present invention found that the lifespan of the cleaning blade or charging roller is shortened when too large an amount of the metal soap is applied to the photoreceptor, and the lifespan of the photoreceptor is shortened when too small an amount of the metal soap is applied to the photoreceptor. Accordingly, it is difficult to simultaneously extend the lifespan of all the photoreceptor, cleaning blade, and charging roller when the metal soap is applied to the photoreceptor as a protective agent.
In the above case in which a metal soap is applied to the photoreceptor, the amount of the metal soap particles present on the photoreceptor is relatively large. Meanwhile, in a case in which a mixture of a metal soap with boron nitride is applied to the photoreceptor, the amount of the metal soap particles present on the photoreceptor is drastically reduced, thus suppressing contamination of the charging roller.
Additionally, in the above case in which a metal soap is applied to the photoreceptor, the metal soap particles are likely to pass through the cleaning blade. Meanwhile, in a case in which a mixture of a metal soap with boron nitride was applied to the photoreceptor, the amount of the metal soap particles passed through the cleaning blade is drastically reduced, resulting in drastic extension of the lifespan of the cleaning blade.
Thus, application of the mixture of a metal soap with boron nitride to the photoreceptor can simultaneously extend the lifespan of the photoreceptor, charging roller, and cleaning blade, while providing high quality images for an extended period of time. The problem is, however, that boron nitride is very expensive.